It is well known in the field of seismic exploration to obtain information about a subterranean earth formation by using sources of seismic energy disposed on the surface of the earth (or in a shallow pit) and seismic detectors disposed at different depths in a nearby well (the synonymous terms "well" and "borehole" will be used interchangeably throughout the Specification). This technique is known as "Vertical Seismic Profiling." lt may also be desirable to employ a downhole seismic source in one well and seismic detectors disposed in another well for seismic exploration.
In seismic exploration, seismic waves generated by the source are reflected by subterranean reflecting surfaces ("reflectors"), and the reflected waves are then detected by seismic detectors such as hydrophones. Seismic detectors have been disposed in a body of water, at the bottom of a body of water, and in wells remote from the seismic source.
One of the limitations of conventional techniques that use monopole hydrophones (i.e., hydrophones that are sensitive to monopole seismic waves) placed downhole in wells is that monopole tube wave arrivals (and to a lesser degree, dipole and higher multipole order modal waves) will dominate their response, obscuring arrivals representing seismic waves that have undergone reflection from a reflector in the subterranean formation. The tube wave and modal wave arrivals, in contrast, represent wave energy that has propagated from the seismic source to the well in which the detector is disposed without necessarily having undergone reflection in the formation, and has then undergone conversion into a tube wave (or other) mode, which mode propagates in the borehole fluid and the formation immediately adjacent the well.
To suppress tube wave noise, downhole wall-locking geophones have been employed in place of monopole hydrophones. These geophones are positioned in direct contact against the borehole wall, typically by elements that may be extended outward to the borehole wall from a sonde suspended in the borehole fluid at the end of a wireline. However, a serious disadvantage inherent in using wall-clamping geophones is the inordinately long time required to position each geophone, lock it against the borehole wall, and s1acken the suspending wireline, which must be done for each receiver location. The speed of acquiring downhole data using wall-locking geophones may be reduced by a factor of fifty or more relative to the speed attainable using non-wall-locking hydrophones.
In conventional marine seismic exploration, hydrophones are typically mounted in elongated cables known as "streamers"Undesired noise often results from acoustic wave energy that propagates generally horizontally to the hydrophones in the streamer, such as seismic wave energy propagating directly through the water from a marine seismic source to the streamer and cable waves which propagate along the streamer. The problem arises due to use of monopole hydrophones which are omnidirectional in the sense that they are sensitive to pressure waves incident from all directions. Omnidirectional receivers are conventionally employed since a marine streamer will unpredictably twist about its axis as it is towed through the water so that unidirectional receivers would be rendered insensitive to vertically propagating waves reflected from subfloor subteranean reflectors at unpredictable moments due to the twisting motion of the streamer.
It has been proposed in U.S. Pat. No. 4,078,223, issued Mar. 7, 1978 to Strange that geophones be mounted in a seismic detector cable and oriented in different directions perpendicular to the cable's longitudinal axis. During operation, the cable is dragged along the earth surface or the submerged floor of a body of water in such a manner that thc cable may twist about its axis when in operation. The geophones that happen to be vertically oriented at any given moment are sensitive to vertically propagating seismic waves but not to horizontally propagating seismic waves. The geophones that happen to be horizontally oriented at a given moment are disabled (such as by gravity operated switches or other electrical or mechanical arrangements) so as to be insensitive to seismic waves. The disadvantages of the Strange system for marine seismic exploration include the following. The frequency response of geophones of the type employed in the Strange system is not flat in the range of interest, and instead is limited to the range of about 10 to 200 Hz. Further, the Strange system does oot detect both vertically and horizontally propagating seismic waves. Finally, the Strange system could not determine the amplitude and incidence angle of non-vertically propagating seismic signals of interest.
Until the present invention it has not been known that a streamer that houses at least one pair of orthogonal dipole hydrophones can be employed to eliminate both the above-recited disadvantages of marine seismic exploration, and the above-recited disadvantages of land seismic exploration operations in which seismic receivers are deployed downhole in a well.